1. Field of the Invention
The present invention relates to a method of depositing glass soot to form an optical fiber preform, and more particularly a method of varying the flame velocity of a glass soot depositing burner.
2. Technical Background
Water in an optical fiber is a source of undesirable attenuation of a light signal propagating along the fiber. Water as used here includes H2O, OH, or H molecules. The silica (SiO2) can react with one of the above forms of water (H2O, OH, or H) to form SiOH which absorbs light strongly at 1380 nm and causes the aforementioned attenuation. The SiOH group in the optical fiber may degrade not only the attenuation performance of optical fibers operating in the 1310 nm window, but may also increase the attenuation of optical fibers operating at wavelengths as long as 1550 nm.
Prior attempts to remove water from the optical fiber include drying soot regions of the optical fiber preform with a halide gas (such as Cl gas, for example) prior to consolidating the preform and drawing the optical fiber from the consolidated preform. Typically, the aforementioned drying takes place at temperatures of about 800-1200° C. The halide gas may be exposed to both an outer surface of the preform as well as a soot centerline of the preform.
However, in the course of manufacturing segmented core fibers using multi-step processes, the aforementioned drying process in some circumstances may be insufficient to reduce the SiOH concentration in the consolidated glass regions of the preform to an acceptable level.
In a modern conventional optical fiber manufacturing process, such as an outside vapor deposition process (OVD), optical fiber may be manufactured by first forming a core cane. By core cane what is meant is a solid glass rod which is an optical fiber precursor comprising at least a portion of the core glass for an optical fiber which may eventually be drawn from the core cane. In some cases the core cane may further comprise at least a portion of the cladding glass for such an optical fiber. In subsequent steps, additional glass is formed on the core cane to form a draw preform. Such additional glass may comprise additional core glass, cladding glass, or both core and cladding glass. The draw preform may then be drawn into an optical fiber. A multi-step manufacturing process advantageously provides significant manufacturing flexibility, as a core cane may form the basis for multiple optical fiber designs and is easily stored for subsequent use, as needed. In a multi-step process, one or more additional layers of glass may be formed on a core cane in one or more steps. The additional glass may be formed on the core cane by heating and collapsing one or more glass tubes over the core cane (sleeving), by depositing glass soot (deposition) onto the core cane and heating and consolidating the glass soot, or both sleeving and deposition/consolidation. As previously stated, the additional glass may include additional core glass, cladding glass, or both core and cladding glass.
When deposition is used to add glass within or adjacent to the core region of an optical fiber preform, the additional layers of soot may form multiple core segments. The refractive index of the segments may vary within each segment, or the refractive index may vary from one segment to another segment. A multi-step process, such as the one described supra, is particularly well-suited to the manufacture of such segmented core optical fibers, and is described in U.S. Pat. No. 4,453,961.
Rewetting of the core region of an optical fiber preform is a significant consideration in the manufacture of low loss optical fibers when employing the combustion of a hydrogen-containing fuel during the deposition process. Rewetting is an especially troublesome issue for the manufacture of optical fibers manufactured with a multi-step process, including, but not limited to, segmented core optical fibers. By multi-step process what is meant is a process of manufacturing an optical fiber preform wherein a glass rod, or core cane, is first made by a conventional process, and which core cane may serve as the target rod for a subsequent deposition of glass soot to form either a next segment of the core, or, optionally, cladding soot may be deposited onto the core cane. Because the predominant portion of the optical power propagating in a single mode optical fiber travels within the core region of the optical fiber, and the distribution of that power is heavily weighted toward the center of the core region, rewetting of the initial glass core rod may significantly affect the attenuation of the optical fiber by placing a high concentration of water in the region of the optical fiber having a high optical power level. Rewetting of the core cane by the deposition of glass soot onto the core cane may lead to a significant increase in optical loss of the resultant optical fiber. To minimize optical loss, or attenuation, in an optical fiber it is preferred that the amount of OH adsorbed into the surface of the glass core cane is minimized. The OH content of the glass may be characterized by measuring OH concentration at a plurality of locations across the radius of a glass rod using Fourier Transform Infrared Spectroscopy (FTIR). The measurement of OH concentration in glass by FTIR is well known. It should be noted that the impact of OH content in the optical fiber is a function not only of the peak amount of OH present at the glass surface, or interface, but also the radial extent of the OH concentration. To that end, as used hereinafter, the term surface, or interface shall mean a region extending about 100 μm from the outermost surface of the glass target, or core cane.
Surprisingly, although the concentration of water vapor at the glass-soot interface during the deposition of soot onto the glass rod may be significant, it is the temperature at the glass surface that exerts the greatest influence over the amount of water adsorbed into the glass. Thus, controlling the glass surface temperature becomes a principal consideration during the deposition process. It has also been discovered by the inventors herein that, due to the low thermal conductivity of glass soot, a relatively thin layer of glass soot deposited on the surface of the core cane is capable of insulating the core cane, thereby reducing the surface temperature of the core cane and limiting the adsorption rate of water into the core cane.
One method that may be employed to decrease the manufacturing cost of an optical fiber preform is to increase the deposition rate of glass soot. Achieving an increased deposition rate has lead to widespread use of multiple soot-producing burners. Although the use of multiple burners to deposit glass soot has produced the desired increases in deposition rates, the high temperature produced at the surface of the glass core cane may undesirably increase the amount of water adsorbed into the glass. Single burner deposition, although typically employing a similar flame temperature as multiple burner deposition, tends to produce a lower surface temperature than multiple-burner deposition. As a single burner flame traverses the length of a glass rod, the localized surface of the rod adjacent the burner flame experiences a period of time between passes of the flame where it cools. The cooling reduces the adsorption of water into the surface of the core cane. The reciprocating relative motion between the burner and the core cane produces a periodic heating and cooling cycle which forms an envelope representing the overall temperature of the glass rod as a function of time. The temperature envelope for a single burner deposition process is typically lower than the temperature envelope for a multiple burner deposition process.
It has previously been assumed that a significant source of water in an optical fiber resulting from a multi-step manufacturing process such as the one described supra, wherein one or more layers of glass soot are deposited onto a glass core cane, originated from incomplete drying of the soot regions of the composite optical fiber preform during subsequent steps to dry and consolidate the preform. It was believed that this residual water migrated to the core region of the preform during the consolidation heat treatment. However, it has been discovered by the inventors herein that a significant source of water which is incorporated into the glass core cane during the subsequent deposition of glass soot originates from the oxidation of the hydrogen-based fuels typically used to hydrolyze the glass soot precursors. The water thus formed may then be deposited on the surface of the core cane. Moreover, the inventors herein have also discovered that adsorption of water into the core cane, resulting in rewetting of the core cane, is dependent upon certain process parameters during the deposition of soot onto the core cane. In particular, the localized temperature of various regions of the core cane and the time during which these localized regions are at a specific temperature play an important part in the amount of water which may be adsorbed. Water which may be adsorbed into the core cane in this manner may not be adequately removed from the preform during drying or consolidation of the preform, and may therefore remain in the drawn optical fiber. The adsorbed water may react with silica to form SiOH, which has a broadband absorption at about 1380 nm, and which in turn may result in an increased attenuation in an operating wavelength range, or window, used within the telecommunication industry.